The present invention relates to a differential input circuit, and more specifically to the technology for broadening a common mode operation range of a differential input signal.
The differential input circuit having a differential signal as an input, can be conventionally realized by differential pairs using field effect transistors (FET) and bipolar transistors. The conventional differential input circuit has a limitation in the input common mode operational range of the differential input signal because of a gate and source voltage V.sub.gs of the FET and a base and emitter voltage V.sub.be of the bipolar transistor both constituting the differential pair.
Accordingly, in order to improve the limitation, there is provided a method for broadening an operational range of a common mode potential in an input differential signal by means of a current addition between outputs of two pairs of differential pairs having different conductive types, as disclosed in Pardoen, "A Rail-to-Rail Input/ Output CMOS Power Amplifier", IEEE JSSC Vol. 25 No. 2 pp. 501-504 (see FIG. 1 of the IEEE document).
However, in this method, since there is a case in which where one of the differential pairs directly operates and the other of the differential pairs performs practically no operation, and a case where both of the differential pairs simultaneously operate because of the largeness of an input common mode potential, there is a problem that a sum of the output currents obtained by the differential is uneven in accordance with an operating point of an input common mode potential caused by a matching error of the current source in each of the differential pairs. Therefore, when the differential amplifier circuit is used in a circuit having a resistance load, an output operating point becomes uneven.
As described above, in the conventionally used differential input circuit, the sum of the output currents for obtaining the differential signal is uneven depending on the common mode input operating point, thereby resulting a problem that a stable output cannot be obtained.